Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.
The profile (vertical cross section) of a knife blade and the way in which this profile varies along the length of the blade determine the performance and longevity of that knife blade. For this reason many different profiles have been used, each with advantages and disadvantages.
FIG. 1 shows, in cross section, a plain carbon steel profile often used prior to the requirement for stainless steel blades in the food processing industry, where the food industry is intended to cover meat, poultry, fish, vegetable etc processing. Stainless steel was required for health reasons, plain carbon steel rusts in these environments and can contaminate the food products produced.
FIG. 2 shows, in cross section, a stainless steel blade which has an edge bevel (edge) that is ground to a different, normally, wider angle than the body of the blade. This wider angle is required due to the properties of stainless steel. It is not practical to grind the whole blade to this wider angle as the blade inward of the cutting edge would be thicker than is necessary, which would increase the amount of work required to move the knife through the material being cut.
To further reduce the drag of the blade behind the cutting edge a variety of knife profiles are used, one of these is termed ‘hollow ground’. FIG. 3 shows, in cross section, a hollow ground knife. These knives have a section of the blade adjacent to the cutting edge ground out of the blade, most commonly with a small diameter (3″ or 4″) grinding wheel, this leaves a raised shoulder between the inner extremity of the cutting edge and the hollow ground section. This shoulder is intended to provide support for the cutting edge. The hollow ground section runs parallel to the peripheral edge of the cutting edge. The hollow ground section reduces the thickness of the blade close to the cutting edge and thus reduces the amount of force needed to use the knife. Unfortunately, as each side of the blade is hollow ground, it creates a thinner section of the blade above the shoulder, this thin section is a weak point. In some cases, when processing for example beef, this thinner section can collapse under normal use, even with the shoulder present.
As a hollow ground knife wears and is resharpened/reground the cutting edge moves into the hollow ground section removing the supporting shoulder and increasing the likelihood of collapse, it also increases the work required to use the knife. The amount of work needed to use the knife in this state can be reduced by grinding the knife with a replacement hollow ground section and remaking the edge, of course this can only be done so often before the blade requires replacement.
In U.S. Pat. No. 2,566,112 H. W. Barnard discloses a knife blade with two parallel adjacent hollow ground sections aimed at providing the advantages of a hollow ground blade during the lifetime of the knife. This type of blade requires two hollow ground sections, each with different radii, are produced in the blade and does not overcome the potential weak sections of blade created by the hollow grind itself. This type of knife blade will also potentially collapse unless carefully manufactured, and maintained. Though with the two hollow ground sections it is uncertain what would happen as the knife was reground/resharpened to maintain the edge.
U.S. Pat. No. 4,495,698 describes what it calls a ‘concave grind’ which is similar to a hollow grind but uses a larger radius grinding wheel. One difference over the prior art is that the concave grind does not follow the cutting edge, it runs in a substantially straight line along the length of the blade (Col 2. lines 35 to 41), it does not follow the edge, this provides a blade with the properties of a flat wedge knife and a hollow ground knife. This ‘concave grind’ is said to avoid the problems associated with a hollow ground knife or very thin knives that tend to break. The concave grind is intended to provide the thin blade cutting properties with the strength and rigidity of a thick blade. U.S. Pat. No. 4,495,698 indicates that the profile essentially follows the surface of a 30″ grinding wheel, using the 8″ Cooks knife example and calculating the thickness of a blade made this way the blade, as ground, would be 15 thou (0.38 mm) at the edge, 17 thou (0.43 mm) at 1/16″, 19 thou (0.49 mm) at ⅛″, 24 thou (0.61 mm) at ¼″, 29 thou (0.74 mm) at ⅜″ about 40 thou (1 mm) at ½″ and 80 thou (2 mm) at 1.25″, and table 1 gives 15 thou at the edge, 40 thou at the midpoint (about ½″ up) and 80 thou at the back (though this may just be the blank thickness). For succinctness we use the standard shortened form of thousandths of an inch, thou, for thickness measurements (where 1 thousandths of an inch is 0.0254 mm). U.S. Pat. No. 4,495,698 specifically has an object of cutting through the entire thickness of crisp vegetables, and to accomplish this objective the inventor has created a blade with the side faces slightly concave over their entire width. This concave grind is said to give the cutting characteristics of a thin blade with the strength and rigidity of a thick blade which is desirable and overcomes the problems with very thin blades for kitchen use. Certain types of knives, for example boning and filleting knives depend on the flexibility of the tip to manoeuvre around bones, with the profile described in U.S. Pat. No. 4,495,698 running straight along the length of the blade the back needs to be ground to achieve this. With an as ground thickness of 15 thou the finished blade edge thickness will be less than this, this is unlikely to be suitable for some applications.
When a knife blade is blunt, or has been damaged it may need to be reground to reset the cutting edge, this is risky process as around 50% of knife wear comes from incorrect aftermarket grinding.